1. Field of the Invention
The present invention relates generally to a mobile communication system for supporting an enhanced uplink dedicated transport channel service. In particular, the present invention relates to a method and apparatus for scheduling uplink packet transmissions from User Equipments (UEs) based on information received from a Serving Radio Network Controller (SRNC) in a Node B.
2. Description of the Related Art
An asynchronous Wideband Code Division Multiple Access (WCDMA) communication system uses an Enhanced Uplink Dedicated CHannel (E-DCH). The E-DCH was designed to improve the performance of uplink packet transmission in the WCDMA communication system. New techniques have been introduced to the E-DCH transmission, including Adaptive Modulation and Coding (AMC), Hybrid Automatic Repeat reQuest (HARQ), and shorter Transmission Time Interval (TTI). AMC and HARQ are existing schemes adopted for High Speed Downlink Packet Access (HSDPA). A TTI is a time unit in which one data block is carried on a physical channel. In HSDPA, a Node B (instead of a Radio Network Controller (RNC)), is responsible for uplink scheduling as well as downlink scheduling. Accordingly, the uplink Node B-controlled scheduling differs from the downlink Node B-controlled scheduling.
FIG. 1 illustrates uplink packet transmission on the E-DCH in a typical mobile communication system.
Referring to FIG. 1, reference numeral 100 denotes a Node B supporting the E-DCH service and reference numerals 101 to 104 denote UEs using the E-DCH. The Node B 100 schedules E-DCHs for the UEs 101 to 104 based on their channel conditions. The scheduling is carried out such that a lower rate is allocated to a UE that is remote from the Node B 100, and a higher rate is allocated to a nearby UE to avoid a noise rise measurement at the Node B 100 exceeding a target noise rise.
FIG. 2 is a diagram illustrating a signal flow for a typical E-DCH transmission and reception procedure between a UE 202 and a serving Node B 201.
Referring to FIG. 2, the Node B 201 and the UE 202 set up an F-DCH between them in step 203. Step 203 involves message transmissions on dedicated transport channels. The UE 202 transmits scheduling information to the Node B 201 in step 204. The scheduling information may contain the transmit (Tx) power, the Tx power margin, or the amount of buffered transmission data of the UE 202. The uplink channel status of the UE 202 can be estimated from the Tx power and the Tx power margin.
In step 211, the Node B 201 monitors scheduling information from a plurality of UEs to schedule uplink data transmissions from the individual UEs. How the scheduling is performed may vary with the Node B 201, which will be described in greater detail below. If the Node B 201 decides to approve an uplink packet transmission from the UE 202, it transmits scheduling assignment information, i.e. a scheduling grant to the UE 202 in step 205.
In step 212, the UE 202 determines the Transport Format (TF) of the E-DCH based on the scheduling assignment information. The UE 202 then transmits control information about the E-DCH and E-DCH data to the Node B 201 at a data rate and a transmission timing determined according to the scheduling assignment information in steps 206 and 207.
The Node B 200 checks for errors in the E-DCH control information and the E-DCH data in step 213. In the presence of errors in either of the E-DCH control information and the E-DCH data, the Node B 201 transmits a Negative ACKnowledgement (NACK) signal to the UE 202 on an ACK/NACK channel, whereas in the absence of errors in both, the Node B 201 transmits an ACK signal to the UE 202 on the ACK/NACK channel in step 208.
The Node B 201 determines a data rate for the UE 202 by scheduling based on the scheduling information received in step 204. The Node B 201 must allocate appropriate data rates and transmission timings to the plurality of UEs. For this purpose, the Node B 201 allocates resources to the UEs by performing scheduling such that uplink Rise over Thermal (RoT) at the Node B 201 does not exceed a target RoT. Accordingly, more resources are allocated to a UE in a good channel condition in order to improve overall system performance.
Now a description will be made of a procedure for scheduling E-DCH transmissions from UEs in the Node B. As stated above, the Node B 201 performs scheduling such that the RoT of the Node B 201 does not exceed the target RoT and such that capacity is maximized as well. The scheduling is based on the scheduling information received from the UEs in step 204 of FIG. 2. The scheduling information is signaled to the Node B 201 as follows.
One method of signaling the scheduling information comprises steps such that each UE signals its Tx power to the Node B 201. The UE may additionally signal a queue size indicating the amount of data buffered in its buffer. The Node B 201 estimates the uplink channel status of the UE from the Tx power, to thereby allocate appropriate resources to the UE.
This signaling method will now be described in greater detail with reference to FIG. 1. The UEs 101 to 104 are separated from the Node B 100 by different distances. The UE 101 is nearest and the UE 104 is farthest. Thus, the UE 101 transmits an uplink channel at the weakest power level, whereas the UE 104 transmits an uplink channel at the strongest power level. Accordingly, to achieve the highest performance under the same RoT measurement, scheduling is done so that power is inversely proportional to data rate. That is, the Node B 100 schedules uplink data transmission in the manner that allocates a higher data rate to the nearest UE 101 with the lowest transmit power, and a lower data rate to the farthest UE 104 with the highest transmit power.
The above-described scheduling is called maximum Channel-to-Interference (C/I) scheduling. However, if each UE signals channel information only, the Node B may lose flexibility in scheduling due to the absence of information about the Tx power margin of the UE.
Even though many resources are allocated to a UE in a good uplink channel status, if the UE does not have a sufficient power margin, it cannot utilize the allocated resources fully. For example, since the UE 101 is near to the Node B 100 and thus can transmit data at a low transmit power level, the Node B 100 can allocate a relatively high data rate to the UE 101. Yet, if the UE 101 does not have a sufficient transmit power margin, full utilization of the allocated resources is impossible. That is, because the Node B 100 has no knowledge of the available Tx power margin of the UE 101, it cannot make an effective decision as to how many resources are to be allocated to the UE 101.
Another method of signaling the scheduling information comprises steps to signal the Tx power margin of the UE as the scheduling information. After receiving Tx power margins from a plurality of UEs, the Node B allocates resources to the UEs by scheduling based on the Tx power margins in the manner that increases cell performance.
This signaling method also has a distinctive drawback in that the Node B cannot accurately estimate the channel condition of each UE. The uplink channel status of the UE cannot be derived accurately from the Tx power margin information only. As a consequence, the C/I scheduling scheme based on channel condition is not viable.
For instance, when the UEs 101 to 104 signal their Tx power margins to the Node B 100, the Node B 100 allocates more resources to a UE having a greater Tx power margin, and less resources to a UE having a smaller Tx power margin. However, when a UE has a sufficient transmit power margin but is placed in a bad channel condition, the Node B does not actually allocate as many resources as corresponding to the Tx power margin. Even if the Node B does allocate such resources, the bad channel condition leads to failed data transmission and reception, thereby decreasing channel capacity.
Accordingly, a need exists for a system and method for effectively and efficiently signaling UE information for use in uplink packet transmission in a mobile communication system.